Since Symfony 2.5.3 update, any form validation doesn't work and gives the following error on PHP < 5.3.8 (I know, that's a shame to use such old versions):

Attempted to call method "getValidator" on class "Symfony\Component\Validator\ExecutionContext" in /var/www/beegreen_recetteprojet/sf/vendor/symfony/symfony/src/Symfony/Component/Form/Extension/Validator/Constraints/FormValidator.php line 56.

I will give as much information as I can on this issue so that it can be reproduced and be fixed.